By Barbara Donohue
Today’s Machining World Archives September 2010 Volume 06 Issue 07
Composites and graphite require a new way of thinking about machining.
You might not normally think of making things out of carbon, but lately the material of the future is carbon fiber reinforced polymer (CFRP) as used in the new Boeing 787 airliner. In a whole different approach to commercial aircraft design, body sections of composites act as both skin and structure.
Workpieces made of graphite, another form of carbon, are machined for use in many industries. Graphite’s electrical conductivity and high temperature capability suit it for a wide range of applications.
A composite material is made up of two or more different materials that keep their individual characteristics. For example, fiberglass contains a plastic material reinforced with glass fibers. Many metal/metal composites or metal/ceramic composites are also in use, but won’t be covered here.
In a fiber reinforced composite, the geometry of the reinforcement fibers, whether parallel, random, or woven and the proportions of fiber and resin (epoxy or various plastics) can tailor the mechanical properties of the material to a particular application. In high stress areas, different configurations of materials, like layered sandwiches, provide added strength.
In an aircraft such as the 787, composites’ high strength-to-weight ratio allows designers to minimize the weight of the craft, reducing fuel consumption. CFRP and other composites also generally require less frequent maintenance, and this reduces upkeep costs. Over the life of the airplane, the savings could add up to the original cost of the plane.
Composites also fit well in such large precision components as wind turbine vanes.
When machining graphite and CFRP, you’ll need to take into account their special characteristics.
First, rather than cleanly shearing as metal will usually do these materials fracture, leaving you with fine graphite powder or little chunks of plastic and carbon. The powder from graphite or small shards of carbon fiber is abrasive and can quickly wear cutting tools.
These materials generally have a higher coefficient of thermal expansion than most metals, so they can shrink and grow more than you’d expect with changes in temperature. In precision work, especially, you must work in climate-controlled space and allow parts or blanks to come to room temperature before cutting or inspecting.
Because these materials are somewhat porous, you need to cut them dry, so they won’t absorb coolant which can cause problems later on.
These materials are expensive. A 12-inch square of CFRP an inch thick might cost $1400, or 40 or 50 times the cost of a similar-sized piece of graphite and 150 times the cost of aluminum stock, said Peter Guercio, vice president of sales and application support at Graphite Machining Services, Inc., Tempe, Ariz.
Composites, by their nature, are created at near net shape and often the only machining required is a bit of milling along the edges and drilling holes. Because the parts can be quite large, too large for available machining centers, much drilling has been done by hand, said Kevin Mayer, aerospace manager at Sandvik Coromant, Fair Lawn, N.J.
Graphite material and applications
Graphite is a form of carbon. It occurs in nature, but the type of graphite used in industry is manufactured to exacting specifications.
“What we use is petroleum coke, a byproduct of oil refining,” Guercio said. The process, which takes three to six months, starts with superheating and cleaning the petroleum coke, mixing it with binders and then subjecting it to heat and pressure. Then the material is graphitized, a heat treating process that changes the material’s structure.
Graphite is available in many forms and grades for different applications. Machining parts with a good surface finish requires a fine-grained grade of graphite, for example.
The blocks of graphite his shop starts with are 12” x 20” x 40”. “Basically, you’re dealing with a coffin that weighs 1200 or 1300 pounds,” Guercio said. The first step in machining is to saw the blocks into pieces.
Applications for graphite cover many industries. Graphite crucibles take advantage of high temperature capability. EDM tools take advantage of graphite’s electrical conductance and stability. Bearings and bushings make use of graphite’s low wear rates and lubricity.
Many industries use graphite parts, including semiconductors, glass working, electronics, electrical and mining/refining. “I’m actually shocked what people use graphite for,” Guercio said.
Composite materials and applications
The fiber or fabric reinforcing material, pre-impregnated with epoxy or other polymers, known as “prepreg,” is laid into a mold of the desired shape. It is cured, often under the pressure and heat of an autoclave. The mechanical properties of a composite depend on its components, their proportions, and the process by which the part is assembled and cured.
The many kinds of composites have innumerable applications. The carbon composites discussed here have been used for decades in military aircraft and now in commercial craft and ground-based vehicles. The place many people encounter carbon composites is in recreational equipment like tennis rackets, golf clubs, masts for windsurfers, bicycle frames.
Machines for graphite and CFRP
Since you shouldn’t use coolant with these materials, you may want to use a cold air gun, which can both cool the tool and remove the dust from the cutting area.
When machining graphite, you need to keep three things in mind, said Bill Howard, product line manager for vertical machining centers at Makino, Mason, Ohio.
First, you’ll need a fairly high RPM spindle. Graphite cuts freely, so it’s pretty much wide open —you can cut it as fast as you want. Makino offers 20, 30, 40 thousand RPM machines. Horsepower and torque are not that big of a concern. Since cutting forces are small, he said, you can have aggressive feed rates.
Second, you need to have a control capable of extremely precise feeding. It’s one thing to have a car go fast if you’re going in a straight line. But if you’re contouring in 3D your control needs to be able to control overshoot and under-shoot. “Once you start spinning the spindle at high RPM, you need to have control over the feeding,” Howard said.
Lastly, you need to be able to handle the dust and keep it away from the operator and the workpiece. “The concept we use is to put a slight vacuum in the work zone and pull air in at the top of the operator door and across the work zone,” said Howard, creating turbulence so the dust doesn’t settle on table or workpiece.
“It’s a dirty business,” he said. “Mostly we have point-of-contact collection, as the graphite is being cut. We create custom dust collectors – at the tool or the fixture. ” The dust collectors have HEPA (high efficiency particulate air) filters to catch the extremely fine dust particles. “Particles of graphite tend to destroy your machine,” said Guercio. They’re abrasive. They conduct electricity.
Makino offers a number of vertical machining centers designed for graphite, and a multipurpose vertical machine, the S56, which, with a special graphite package, can machine graphite.
Tooling for graphite, CFRP, and other composites
Due to its abrasiveness, graphite can quickly wear down tools. For precision work, you should use carbide tools with appropriate coatings, Guercio said. “Make sure you use tools from well-known manufacturers,” he said, “or the coating will crack right off when you machine “graphite.”
You have to deal with multiple materials when you cut composites. Tool manufacturers have developed specialized drills and other tools to help alleviate some of the problems. Drilling presents particular challenges.
With sandwich composites, you may be drilling through layers of vastly different materials, such as titanium and CFRP. The CMD drill from Precorp, Spanish Fork, Utah, is designed to give consistent hole size from one layer to the next through different materials, said company president Rich Garrick.
Drilling CFRP may result in separation of the layers of material—delamination—or splinters of fiber may break out. Sandvik Coromant offers drills to prevent these problems. For composites with relatively high fiber content, splintering tends to be a problem. The CoroDrill 854 can alleviate this problem with extra cutting edges that sever the fibers on the hole diameter. For resin-rich composites, Sandvik Coromant offers the CoroDrill 856 designed to alleviate delamination problems.
Polycrystalline Diamond (PCD) coatings or edges help tools to last. Crystallume and Precorp have processes for incorporating a section of PCD in a tool just at the cutting edge.
Tricks of the trade
Shops experienced with machining these challenging materials have learned how to tame them. “Graphite is black magic, no pun intended,” said Guercio.
“You really need to know what you’re doing. Err to the side of caution, especially when tolerances are a concern. Take your time, measure your part, and gradually ramp up your feeds and speeds until you optimize [the process].”
When drilling through a part made up of layers of radically different materials, you should program different spindle speeds for each material.
Because graphite and composites wear tools so fast, you’ll probably want to monitor and manage the tools to keep good product coming off the machine. Howard suggested using laser automatic tooling measurement. The spindle periodically moves the tool into the path of the laser, which can not only measure the length and diameter of the tool as it spins, but some systems can detect the tool’s form and trigger a tool change if it is worn or if an edge is chipped.
More to come
With development of advanced composites, you’ll be seeing more and more of them showing up in many applications. Even if a part comes in made from metal, it’s quite likely the customer has looked at making it in composite, Mayer said. “There is this tidal change taking place. What’s really happening here is being driven through these more flexible, more tolerant, lightweight materials.”